1. Field of the Invention
The present invention relates generally to semiconductor devices and particularly to those comprising a redundant circuit including a fuse.
2. Description of the Background Art
As semiconductor devices are increasingly microfabricated, foreign matters have a significant effect on yields of semiconductor devices. In semiconductor memory devices having memory cells of dynamic random access memory or the like a redundant circuit is used to connect a previously formed, spare, defectless cell in place of a specific cell defective due to foreign matters or the like.
To substitute the defective cell with the defectless cell a fuse provided in the redundant circuit would be blown. The fuse is typically an interconnection layer formed in a portion corresponding to an upper layer of the semiconductor memory device.
Generally a fuse is blown by laser-trimming using a laser beam. The laser beam directed to a specific fuse to illuminate the fuse blows it.
As a first arrangement in a redundant circuit at a portion provided with a fuse an insulation film is simply disposed between the fuse and a semiconductor substrate to prevent blowing the fuse from affecting interconnections, semiconductor elements and the like otherwise arranged in a region immediately underlying the fuse and a region located in a vicinity thereof.
As a second such arrangement a blocking layer is disposed between a fuse and a semiconductor substrate to prevent blowing the fuse from affecting an underlying layer.
In particular, semiconductor devices corresponding to the second arrangement are proposed for example in Japanese Patent Laying-Open Nos. 11-345880, 2000-114382, 2000-68377, 10-242280, 10-294372, 2-25055, 63-3432 and 9-17877.
The above conventional semiconductor devices, however, have the following disadvantage: in recent years, there is an increasing demand for system large scale integrated circuits (LSIs) as semiconductor devices. For a system LSI a fuse overlies six or more layers for example.
A fuse overlying larger numbers of layers would have larger distances from a surface of the semiconductor substrate. Accordingly for the semiconductor device having the first arrangement a component of a laser beam radiated to blow the fuse that is transmitted through a layer underlying the fuse and reflected back by the surface of the semiconductor substrate would spread over a wide range remote from the location of the fuse.
As such, the reflected laser beam illuminates and negatively affects a semiconductor element, an interconnection and the like formed in a vicinity of the fuse.
Furthermore for the semiconductor device having the second arrangement the blocking layer disposed immediately under the fuse would regularly reflect the laser beam. As such, it is possible that a laser beam illuminating one of a plurality of fuses that is arranged at an end, and reflected by the blocking layer, may affect an interconnection, a semiconductor element or the like existing in a vicinity of the illuminated fuse.
Consequently, semiconductor elements, interconnections and the like cannot be arranged adjacent to a region having a fuse formed therein, which is an obstacle to reduction in size of semiconductor devices.